1. Field of the Invention
The present invention relates to an exhaust gas controlling catalyst that uses a base metal as an active metal, a method for the production of a carrier for the catalyst, a method for the production of the catalyst, and an exhaust gas control apparatus that uses the catalyst.
2. Description of Related Art
In the field of motorcars and so on, exhaust gas controlling catalysts that use a noble metal, such as Pt, Rh or Pd, as a catalyst metal are used. In contrast to this, exhaust gas controlling catalysts that use a base metal, instead of a noble metal (catalyst metal), as an active metal to reduce material costs are under development.
Compared to noble metals, base metals are much less active as active metals, and are inferior in ability to reduce NOx, in particular, among exhaust gas components HC/CO/NOx in stoichiometric control.
One possible solution is to reduce NOx to harmless components with the air-fuel ratio of the exhaust gas controlled to be richer than the theoretical air-fuel ratio. In this case, the combination of the base metal, such as Cu, and the carrier that supports the base metal as an active metal is critical.
Japanese Patent Application Publication No. 2009-131835 (JP-A-2009-131835) discloses a structure in which nanoparticles of Cu or the like are supported on a powdery carrier in a dispersed manner. The carrier has a powder particle size of 0.1 to 30 μm and is composed of one or more materials including zirconia and alumina. The NOx conversion efficiency decreases at high temperature (500° C. or higher) because the base metal nanoparticles are oxidized by the oxygen that is released from the inside of the carrier when, in particular, nanoparticles of a base metal, such as Cu, are supported on a ceria (CeO2) carrier in a dispersed manner, whereas the NOx conversion efficiency decreases at low temperature (250° C. or lower) when a zirconia (ZrO2) carrier is used.
Ozawa Masakuni: Ceria and Automotive Catalyst, Annual Report of the Ceramics Research Laboratory Nagoya Institute of Technology, vol. 2, 1-8, 2002 discloses that a decrease in conversion efficiency due to fluctuations of the air-fuel ratio can be prevented when ceria is used as a promoter in a catalyst that uses a zirconia carrier. Also disclosed is that the amount of ceria necessary to obtain peak performance is 50 mol % for at 700 K and 30 mol % for at 1300 K.
Toshio Nakatani, Hiroshi Okamoto and Rikuo Ota: Preparation of CeO2—ZrO2 Mixed Oxide Powder by the Coprecipitation Method for the Purification of Automotive Emission, Journal of Sol-Gel Science and Technology Volume 26, Numbers 1-3, 859-863, 2003 discloses a promising carrier for an exhaust gas controlling catalyst that has high temperature stability achieved by adding zirconia to ceria and also has high oxygen storage/release capacity (OSC). Also disclosed is that an oxidation reduction reaction produces a local effect when ceria is present in an amount of 30 to 60 mol % with respect to zirconia.
However, a catalyst that is composed of a base metal, such as Cu supported by a CZ carrier obtained by dissolving ceria in zirconia to improve the heat resistance of the ceria has a lower NOx conversion efficiency at high temperature (500° C. or higher, in particular).